We have devised a new method to measure cytotoxic T cell activities in vitro and perfected a method to measure immune rejection in vivo. using these techniques in combination with our standard thymic organ culture, we are in the process of studying four questions. 1. Do newly born T cells have the thymus in a tolerizable or activateable state. 2. Does thymic epithelium present antigens in a tolerogenic or immunogenic fashion. 3. Does T cell priming and/or memory require the presence of B cells. 4. What is the frequency of alloreactive T cells. Our preliminary experiments suggest that newly developed CD4+ and CD8+ T cells are fully capable of responding to allogeneic stimulators, thus refuting models that require the assumption that T cells leave the thymus is an anergizable state.